1. Field of the Invention
The present invention relates in general to the handling and disposal of waste water effluents of the type produced during the manufacture of printed circuit boards and, in particular, to a method for removing aqueous photoresist waste from a waste water stream prior to discharging the waste water stream. Still more particularly, the present invention provides an improved method for removing aqueous photoresist waste through acid precipitation that does not produce a sticky precipitate.
2. Description of the Prior Art
Several techniques are known in the prior art for manufacturing printed circuit boards in which a light sensitive substrate is "exposed" to create a desired pattern. For instance, in one such technique, a board having a photoresist film covering a copper surface is masked such that the desired pattern is exposed to ultraviolet light. The board is developed in a sodium carbonate or potassium carbonate system stripping the unexposed photoresist film. As a result, the unexposed sections have no film covering the copper. Next, the board is etched to remove the exposed copper areas and then, the board is stripped in a sodium hydroxide or potassium hydroxide solution to remove the remaining photoresist film.
A developer solution and a stripping solution are used to remove the photoresist film from the board. Considerable interest exists regarding the handling and disposal of these waste water effluents in attempting to comply with increasingly restrictive environmental regulations and sewering codes. Since these photoresists are an integral part of the circuit board manufacturing process, the solutions generated from the developing and stripping of the photoresists become a part of a waste stream that often needs to be treated before discharging this waste stream, for example, into the local sewer system.
In many instances, it is desirable to remove the photoresist from the waste stream. Removal of the photoresist may involve acid treatment and carbon absorption. In acid treatment, acid is added to the waste stream to decrease the pH which causes the photoresist to precipitate out of solution. One problem with this acid treatment is that a gummy, sticky precipitate may occur. Such a precipitate cannot be clarified, filtered, or pumped by standard means. Therefore, it would be desirable to have a method of acid treatment that would not produce a gummy, sticky precipitate.